I have noticed that the mains don't lose tension but the crosses drops quite a bit after several hours of hitting. So if I am using a 1.20 co-poly, would it be better if I do the cross strings with a 1.25 of the same string so tension maintenance is better? Also will this give me more spin due to the different thickness of strings from mains and crosses?

The thinner cross will allow your mains to move more freely, which should result in more spin.

If you string, you should also be aware that crosses are harder to pull to reference tension due to string friction as they are pulled. With LO, you may have to pull twice. With DW, straightening the strings and pulling for > 10 seconds is needed. If neither is done, chances are you are below the desired reference tension.

There are people playing with thick/thin, thin/thick, and full beds of the same gauge.

I try different set ups to find what works best for me. I usually prefer thinner crosses. For the mains, it depends on the racket and strings. I may prefer the thick version of one string and the thinner gauge of another.

Best way is to find out for yourself since you have your own stringer. Get a set of your usual strings (or a cheaper alternative if its expensive) in 16g and a set in 17g or 18g, preferably with a large difference between the two sets. String up two identical frames, one with thick mains/thin crosses, and flip it for the other. You'll have the most accurate answer for YOU, that way.